A multi-focal lens typically has multiple regions each having a different optical power. The transition between the regions of different optical power may be either abrupt, as is the case for bifocal and trifocal lenses, or smooth and continuous, as is the case with Progressive Addition Lenses (PALs). Progressive Addition Lenses are a type of multi-focal lens that comprise a gradient of continuously increasing positive optical power from the beginning of a far distance viewing zone of the lens to a near distance viewing zone in the lower portion of the lens. This progression of optical power generally starts at approximately what is known as the fitting cross or fitting point of the lens and continues until the full add power is realized in the near distance viewing zone. Typically, the optical power then plateaus. Progressive Addition Lenses utilize a variable curvature surface on one or both exterior surfaces of the lens that is shaped to create this progression of optical power. Progressive Addition Lenses are advantageous over traditional bifocal and trifocal lenses in that they provide a lineless, cosmetically pleasing multi-focal lens with a continuous vision correction from a far distance to a near distance.
Using PALs, however, presents problems. One problem is that PALs have serious vision compromises that include, but are not limited to, unwanted astigmatism, distortion, and perceptual blur, especially for large near distance corrections (large near add powers). In addition, many wearers of PALs experience an unpleasant effect known as visual motion (often referred to as “swim”) due to the distortion that exists in the lenses.
To minimize these problems, the overall optical power of PALs may be reduced to provide a wearer with a desired optical power less than the wearer's needed near distance optical power correction. The remaining optical power may be provided by a diffractive optical element which generate little or no additional unwanted astigmatism, distortion, perceptual blur, or swim.
While the use of a diffractive optical element in combination with a low power PAL may considerably reduce visual distortion of the PAL (especially for large total add powers), the diffractive lens may itself introduce new unwanted effects, e.g., noticeable chromatic aberrations in higher prescriptions due to the highly dispersive nature of diffractive optical elements.
There is therefore a great need in the art for providing a PAL in combination with a diffractive lens, while minimizing the chromatic aberrations of the diffractive optical element. Accordingly, there is now provided with this invention an improved lens for effectively overcoming the aforementioned difficulties and longstanding problems inherent in the art.